One example of a welded structure of plural parts joined together by welding is a welded structure between, for example, a differential case and a ring gear of a differential device (hereinafter, “differential gear”) of an automobile. FIG. 11 shows a conventional example of a welded structure between a conventional differential case 100 and a ring gear 102. A hypoid gear is formed in a teeth portion 102a of the ring gear 102. In the conventional example of FIG. 11, a groove 104 is provided in the joint surface between the differential case 100 and the ring gear 102. Thus, compressive stress and shear stress on weld beads 105 caused by a load applied in directions shown in the drawing in actual operation of the differential gear are less concentrated on a weld bead end 105a. 
However, in a conventional example in which a helical gear is formed in a teeth portion 106a of a ring gear 106 as shown in FIG. 12, the load is applied repeatedly in directions indicated respectively by solid line arrows and broken line arrows in actual operation of the differential gear. This causes compressive stress and tensile stress to be generated repeatedly at both ends in the central axis direction of the ring gear 106 (up and down direction of the drawing) of the joint surface between the differential case 108 and the ring gear 106. In the conventional example of FIG. 12, weld beads 109 are formed only from one side in the central axis direction of the ring gear 106 of the joint surface between a differential case 108 and the ring gear 106. Therefore one end of the weld beads 109 may be repeatedly subjected to large compressive stress and tensile stress, which may result in insufficient weld strength. Even if a groove 110 is provided in the differential case 108 in its joint surface with the ring gear 106, such groove cannot provide an effect of reducing stresses applied to the end on the groove 110 side of the weld beads 109.
Patent Document 1 discloses a technique in which a flange member is disposed between a small gear wheel member and a large gear wheel member, and the flange member is joined to the large gear wheel member with the outer periphery of the flange member being in contact with the inner periphery of the large gear wheel member such that it is welded from both sides in the central axis direction of the large gear wheel member.